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Pete Docter and Jonas Rivera are consummate professionals. You'll see it in their work – the new Disney/Pixar film “Up,” opening today. But their eyes hint at some of the mischief that went into creating this production.

It is the 10th in an award-winning line from Walt Disney Pictures and Pixar Animation Studios that started 14 years ago with “Toy Story.” Docter, the film's director, and Rivera, its producer, admit they're still kids at heart. And that's probably why the movie is so funny, so wacky, so charming. It's definitely why all the films have worked so well, appealing to kids and adults on their own levels. These guys and their playmates at Pixar have not forgotten the magic of make-believe, the joy of childhood, the focus on friends.

“Up” has all this and, yet, feels a little different from the others. Maybe because it's a love story, too. We meet Carl (Ed Asner) and Ellie Fredericksen in a wordless four-or five-minute sequence that manages to speak volumes. Theirs is a love of adventure that surfaces in their childhood (Docter's daughter, 10-year-old Elie, is the voice for young Ellie, and Jeremy Leary is young Carl) and we watch their lives play out. There is no old Ellie voice, but you'll understand why.

It is when Carl, a 78-year-old retired balloon salesman, has lost so much, and is about to lose his house, that Russell (newcomer Jordan Nagai, 9) shows up at his door. He's an overly eager Junior Wilderness Explorer scout who needs to earn his last merit badge – for helping the elderly.

He's like a gnat irritating Carl, who is part curmudgeon, part softy. Theirs is a hilarious story, an amazing adventure and, deep down, a warm relationship.

Story is everything, says Docter, relaxing recently with Rivera at the rooftop lounge of the Sè San Diego Hotel downtown.

“We spent about 3-½ years on the story,” Docter says.

“The production took about 2-½ years,” adds Rivera. “The top animators will complete about four seconds a week.”

The finished product is 96 satisfying minutes.

“It's like making a movie in slow motion,” Docter explains.

So, guys, were you thinking of your own grandparents when you were working on this film?

“For sure, we all pulled from our grandparents,” says Docter, who added that he spent time with other older people to make Carl's character authentic. That included Joe Grant, a Disney story artist still working in his 90s, who has since died. Docter's own grandparents are gone, too, but memories of them are very much alive and integrated into Carl.

Rivera still has three of his grandparents and studied particularly his grandfather, Ralph Lopez, going strong at 90 in Oakland.

“I put up a video camera and just watched his mannerisms,” says Rivera.

They start with that kind of research. Once they had the story, which opens in the 1930s and progresses for decades, they were off to study locations, including trekking to a remote part of Venezuela so they could adequately reproduce the place Ellie had always wanted to visit. Realism, even the animated kind, is paramount. Attention to detail is enormous, such as a tag sticking out of the collar of someone's shirt, and how dogs behave and move (there are lots of them in the film).